A method of manufacturing a textured screw

Abstract

The application discloses a preparation method of a textured screw and belongs to the field of high-temperature metallurgical connecting structural member preparation. The application is characterized in that long strip beta-silicon nitride whiskers and a wax-based binder are added into heated ceramic slurry, the ceramic slurry is shaped by a low-temperature wax-based auxiliary agent, and tapping is performed on the shaped ceramic slurry by a mold. During the tapping process, a micro-texture effect is formed on the surface of the thread, so that the shear force of the thread part of the prepared screw is higher than that of a conventional ceramic thread, and a certain degree of damage to the ceramic structure is avoided during the processing, so that the quality of the thread part is affected.

Description

Technical Field

[0001] This invention relates to the field of high-temperature metallurgical connection structure preparation, and in particular to a method for preparing textured screws. Background Technology

[0002] Silicon nitride screws and nuts, with their core advantages of high temperature resistance, thermal shock resistance, and chemical inertness, have become an ideal fastener solution for the extreme environments of metal smelting. The combination of their low coefficient of thermal expansion and high hardness ensures dimensional stability at high temperatures while significantly extending equipment maintenance cycles through excellent wear resistance and impact toughness. Furthermore, the lightweight nature and self-lubricating properties of silicon nitride further optimize safety and reliability in high-frequency vibration scenarios, making them particularly suitable for critical components such as continuous casting machines and high-temperature reactors. Although the initial investment is higher, their long lifespan reduces overall lifespan costs, driving the metallurgical industry towards high efficiency and low carbon emissions.

[0003] Traditional ceramic screws and nuts are manufactured by first preparing the screw and nut bodies, and then machining the threads. Because the threads of screws and nuts are very fine, extremely high precision is required during machining, resulting in very high costs. Furthermore, the machining process causes some damage to the ceramic structure, affecting the quality of the threaded portion and leading to a high scrap rate. Summary of the Invention

[0004] The innovation of this invention lies in adding long strips of β-silicon nitride whiskers and wax-based sintering aids to heated ceramic slurry. The ceramic slurry is shaped by a low-temperature wax-based binder, and then tapped by a mold. During the tapping process, a micro-texture effect is formed on the thread surface, which can be divided into the following two points.

[0005] Innovation in green body machinability: A specific type and proportion of wax-based binder is introduced into silicon nitride powder. Through thermal mixing, the binder melts and uniformly binds the powder, which then solidifies upon cooling to form a green body with suitable mechanical strength. This green body has a hardness far lower than sintered ceramics, allowing for high-precision, low-damage thread tapping using conventional carbide taps, completely solving the machining challenges of hard and brittle ceramic threads.

[0006] Structure-performance integrated innovation: Flaky or rod-shaped β-silicon nitride whiskers are introduced into the formulation as texture inducers. These whiskers align under shear force. During subsequent high-temperature sintering, they guide the preferential growth of silicon nitride grains along specific directions, ultimately forming a textured microstructure with anisotropic characteristics.

[0007] A method for preparing a textured screw, characterized by the following steps:

[0008] S1. Preparation of mixed slurry: Silicon nitride powder with α phase content higher than 85%, rare earth element compound, β-silicon nitride whisker powder, dispersant and wax-based binder are mixed and heated and ball-milled until all particles have a particle size of 0.05-1.5μm, thus completing the preparation of hot mixed slurry.

[0009] S2. Place the hot-mixed slurry into the screw mold, cool the hot-mixed slurry to room temperature, and prepare a screw blank without threads.

[0010] S3. At room temperature, the screw blank is processed to produce threads. The processing method is spiral cutting or tapping. During the thread processing, the processing part is processed in a unidirectional spiral manner to produce a thread blank.

[0011] S4. The threaded blank is degreased, sintered, and then processed into a secondary thread to produce silicon nitride screws.

[0012] Optionally, the hot-mixed slurry in step S1 is prepared by hot mixing at 60-80°C for 2-4 hours.

[0013] Optionally, the mass fractions of silicon nitride powder, rare earth element compound, β-silicon nitride whisker powder, dispersant, and wax-based binder in step S1 are 70-90 parts, 2-11 parts, 3-20 parts, 3-12 parts, and 2-6 parts, respectively.

[0014] Optionally, the wax-based binder in step S1 is one, two, or three of paraffin wax and microcrystalline wax.

[0015] Optionally, in step S2, after the hot-mixed slurry is placed into the screw mold, a ceramic rod with an irregular cross-section is inserted along the center line.

[0016] Optionally, after the threads are machined in step S3, they are fixed using a split graphite mold.

[0017] Furthermore, the graphite mold has a threaded structure that matches the screw.

[0018] Optionally, the degreasing process in step S4 involves placing the threaded blank in a degreasing furnace, heating it to 500-600°C at a rate of 0.5-2°C / min, and holding it at that temperature.

[0019] Optionally, the sintering process described in step S4 involves placing the furnace in a nitrogen atmosphere and performing pressure sintering at 1750-1850℃ for 1-4 hours.

[0020] Optionally, step S4 involves secondary thread grinding. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below.

[0022] Figure 1 Here is a structural diagram of the screw;

[0023] Figure 2 This refers to the machining method for the threaded portion of a screw. Detailed Implementation

[0024] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0025] Material Selection: Silicon nitride powder: α-silicon nitride content ≥92%, average particle size 0.8μm, oxygen content ≤1.5%. Sintering aids: Yttrium oxide, MgO, alumina, etc., purity ≥99.9%, average particle size 0.8μm. Texture inducer: Flaky or high aspect ratio rod-shaped β-silicon nitride whiskers, long diameter 5-20μm, short diameter 0.5-2μm. Binder: Paraffin wax, ceramide wax, microcrystalline wax, melting point 40-80℃.

[0026] Formulation design (mass percentage): Silicon nitride powder: 75%-90%, sintering aid: 3%-10% (e.g., yttrium oxide: 2%-6%, MgO: 1%-4%), texture inducer (β-silicon nitride whiskers): 5%-13%, wax-based binder: 1%-4%.

[0027] The powder and molten wax-based binder are mixed in an internal mixer at 65-75℃ for 2-3 hours, and then extruded and granulated to form granules. The granules are heated to a plastic state and injected into a preheated screw blank mold through an injection molding machine or screw extruder. A sintered silicon nitride square prism is inserted at the center line of the mold. The square prism serves as a support in the screw processing, forming a green blank containing the screw shaft and threaded preform, which is then cooled and solidified.

[0028] like Figure 2 As shown, at room temperature, a processing device is used to tap a cured green blank to produce a complete thread. The tapping process is smooth with good chip removal. During the tapping process, the rotary table must maintain a single direction for tapping. As the rotary table rotates and taps, it causes the β-silicon nitride whiskers in the thread green blank to fall horizontally, producing a texturing effect and generating a textured layer on the thread surface. After sintering, this textured layer can provide shear force for the threaded portion. Standard external threads and screw head grooves are precisely formed.

[0029] The processed green blanks were placed in a degreasing furnace and heated to 600℃ with programmed temperature control (0.5-1℃ / min) and held at that temperature to completely remove organic matter. They were then placed in a nitrogen atmosphere sintering furnace and subjected to gas pressure sintering at 1850℃ for 2 hours to finally obtain fully dense textured silicon nitride ceramic screws.

[0030] The two cases are compared below:

[0031] Example 1: The formulation consisted of 83% silicon nitride, 4.5% yttrium oxide, 1.5% MgO, 8% rod-shaped β-silicon nitride whiskers, and 3% composite wax. The mixture was kneaded in a mixer at 70°C for 3 hours to obtain a homogeneous plastic material. The material was then molded into M6×20 screw blanks at 60°C. After cooling and solidification, standard external threads were machined on a precision lathe using carbide cutting tools. Subsequently, degreasing was performed, and the screws were sintered at 1800°C for 2 hours under a nitrogen atmosphere. After natural cooling, the finished product was obtained.

[0032] Comparative Example 1: The same powder formulation as in Example 1 was used, but without the addition of a wax-based binder. A dense ceramic rod was obtained by dry pressing followed by direct sintering, and then the threads were machined using a diamond grinding wheel.

[0033] project β-silicon nitride percentage paraffin content Axial tensile strength (MPa) Creep strain (%) at 1100℃ / 100h Thermal shock resistance (ΔT_c, water cooling) Fatigue life (number of cycles, axial stress ±450MPa) Example 8 3 880 <0.15 >800℃ <![CDATA[>2.0×10 7 ]]> Comparative example 0 0 800 0.25 400℃ (cracking from the root of the thread) <![CDATA[1.8×10 7 ]]>

[0034] The fatigue life of Example 1 is several times to an order of magnitude higher than that of the traditional post-machining method (Comparative Example 1). This is directly attributed to the fact that the blank machining completely avoids fatal microcracks at the thread root, which is confirmed by fracture analysis at the microscopic level. The addition of whiskers significantly improves both axial tensile strength and stripping torque, demonstrating the strengthening effect of the textured structure on the main force direction of the screw. This invention, through the synergy of "machinable blank" and "axial textured structure," solves the three major pain points of silicon nitride screws: "machining damage, isotropic performance, and high cost," providing a new technical path for its large-scale application in high-reliability fields.

Claims

1. A method for preparing a textured screw, characterized in that: the steps include: S1. Preparation of the mixed slurry: Silicon nitride powder with an α-phase content higher than 85%, rare earth element compounds, β-silicon nitride whisker powder, dispersant, and wax-based binder are mixed and heated by ball milling until all particles have a particle size of 0.05-1.5μm, thus completing the preparation of the hot mixed slurry; the mass parts of the silicon nitride powder, rare earth element compounds, β-silicon nitride whisker powder, dispersant, and wax-based binder are 70-90 parts, 2-11 parts, 3-20 parts, 3-12 parts, and 2-6 parts, respectively. S2. Place the hot-mixed slurry into the screw mold, cool the hot-mixed slurry to room temperature, and prepare a screw blank without threads; S3. At room temperature, the screw blank is processed to produce threads. The processing method is spiral cutting or tapping. During the thread processing, the processing part is processed in a unidirectional spiral manner to produce a thread blank. S4. The threaded blank is degreased, sintered, and then processed into a secondary thread to produce silicon nitride screws.

2. The method for preparing a textured screw according to claim 1, characterized in that: The hot-mixed slurry in step S1 is prepared by hot mixing at 60-80°C for 2-4 hours.

3. The method for preparing a textured screw according to claim 1, characterized in that: The wax-based binder in step S1 is one, two, or three types of paraffin wax or microcrystalline wax.

4. The method for preparing a textured screw according to claim 1, characterized in that: In step S2, after the hot-mixed slurry is placed into the screw mold, a ceramic rod with an irregular cross-section is inserted along the center line.

5. The method for preparing a textured screw according to claim 1, characterized in that: After machining the threads in step S3, fix them using a split graphite mold.

6. The method for preparing a textured screw according to claim 5, characterized in that: The graphite mold has a threaded structure that matches the screw.

7. The method for preparing a textured screw according to claim 1, characterized in that: Step S4 describes a degreasing process in which the threaded billet is placed in a degreasing furnace and heated to 500-600°C at a rate of 0.5-2°C / min and held at that temperature.

8. The method for preparing a textured screw according to claim 1, characterized in that: The sintering process in step S4 involves placing the furnace in a nitrogen atmosphere and performing pressure sintering at 1750-1850°C for 1-4 hours.

9. The method for preparing a textured screw according to claim 1, characterized in that: Step S4 describes a secondary thread processing method, which involves grinding.

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